An Introduction to the Design and Maintenance of Cargo System Pressure Relief Valves on Board Gas Carriers First Published in 1994 by: © Society of International Gas Tanker and Terminal Operators Ltd Fully Revised and Updated in February 1998 ISBN 978 1 85609 163 3 2nd Edition September 1998 SIGTTO The Society of International Gas Tanker and Terminal Operators is a non-profit making organisation dedicated to protect and promote the mutual interests of its members in matters related to the safe and reliable operation of gas tankers and terminals within a sound environment. The Society was founded in 1979 and was granted consultative status at IMO in November 1983. The Society has over 100 companies in membership who own or operate over 95% of the world’s LNG tankers and terminals and over 55% of the world’s LPG tankers and terminals. Notice of Terms of Use While the information given in this document has been developed using the best advice available at the time of publication, it is intended purely as guidance to be used at the owner’s risk. No responsibility is accepted by the Society of International Gas Tanker and Terminal Operators Ltd, or by any person, firm, corporation or organisation who, or which, has been in any way concerned with the furnishing of information or data, the compilation, publication or authorised translation, supply or sale of this guide for the accuracy of any information or advice given herein or for any omission herefrom or for any consequences whatsoever resulting directly or indirectly from the compliance with or adoption of guidance contained herein even if caused by a failure to exercise reasonable care. Printed and bound in Great Britain by Bell & Bain Ltd. Glasgow Published in 2009 by Witherby Seamanship International Ltd. 4 Dunlop Square, Deans Estate Livingston, EH54 8SB United Kingdom Tel No: +44 (0)1506 463 227 Fax No: +44 (0)1506 468 999 Email: info@emailws.com www.witherbyseamanship.com WSIL - 199 Introduction to the Design and Maintenance.indd iWSIL - 199 Introduction to the Design and Maintenance.indd i1/22/2009 4:54:30 PM1/22/2009 4:54:30 PM WSIL - 199 Introduction to the Design and Maintenance.indd iiWSIL - 199 Introduction to the Design and Maintenance.indd ii1/22/2009 4:54:31 PM1/22/2009 4:54:31 PM iii Contents Page No. 1. Introduction 1 2. Requirements for Pressure Relief Valves 1 2.1 General 1 2.2 Valve design requirements 2 2.3 Design and installation 3 3. Types of Pressure Relief Valves 5 3.1 Pilot operated valves 5 3.2 Spring operated pressure relief valves 7 4. Preventative and Planned Maintenance Procedures for Pressure Relief Valves 8 4.1 Scope 8 4.2 Pressure relief valve preventative maintenance philosophy 9 4.3 Pressure relief valve preventative maintenance frequency 9 4.4 Pressure relief valve maintenance and inspection 9 4.5 Maintenance and care of diaphragms 11 4.6 Spare parts 11 5. Emergency Closure of Pressure Relief Valves 11 5.1 Emergency closure 12 6. Operating Problems and Faults 13 6.1 Operating problems 13 6.2 Faults 13 6.3 Chloride stress corrosion cracking 14 7. In-Service Testing 19 Appendix 1: IMO IGC Code - Extracts from Chapter 8 Cargo Tank Vent Systems. 21 Appendix 2: Defi nitions and Terminology Used in Regard to Pressure Relief Valves. 29 Appendix 3: Metal to Metal Seat Valves - Precautions and Hints for Lapping Seats. 29 Appendix 4: Pressure Levels 31 Appendix 5: Fire Activated Pressure Relief Systems 33 References 34 WSIL - 199 Introduction to the Design and Maintenance.indd iiiWSIL - 199 Introduction to the Design and Maintenance.indd iii1/22/2009 4:54:31 PM1/22/2009 4:54:31 PM SIGTTO 10 6. Record sheets should be kept for all valves. 7. Before removing valves, check cargo records and health and safety data sheets for any possible decontamination requirements. 8. Never attempt to lap the valve nozzle to the disc. (see Appendix 3 for maintenance hints for metal to metal seats) 9. Accuracy and repeatability of the set pressure will be impossible to achieve if the test rig does not have sufficient surge volume. This is particularly true with spring operated valves, where leakage may occur at 90% of set pressure and significant simmer at 95% set pressure. A small test volume and feed rate may never be able to push the valve past the simmer point and subsequently the valve will be set at a pressure higher than intended. Therefore it can be seen that it is not sufficient to connect an air hose to the inlet flange of the valve and test in this manner. Figure 5: Recommended Size for Test Rig Vessel Size for Air or Gas 10. There is no accepted standard for surge tank capacity, but Figure 5 gives a graph of surge tank volume related to valve orifice area. It is published by the US National Board of Boiler and Pressure Vessel Inspectors and is generally considered to be very conservative. It can be seen that a 150 mm NB valve, for example, with an orifice area of 113 cm2 (17.5 in2) and a design blowdown of 7% would require a surge tank of (133ft3) 8 m3, a facility that few test shops would have. An advantage of pilot valves is that they only require a small surge volume (in the order of 10 litres), due to the small capacity of the pilot. 11. When the valve is on the test rig, it should be fitted with a bubble tester (figure 6) to enable the seat tightness and the exact point at which it starts to lift to be accurately determined. Seat tightness is generally quoted in bubbles per minute (bpm). For soft seat valves, 0 bpm is the norm, whereas 30 bpm is common for metal seated valves. These figures generally apply at 90% of set pressure. API Standard 527 gives more information on this subject. 12. Valves should always be overhauled and tested in a clean environment, using clean tools and trained personnel. 13. The valve discharge nozzle must be positioned to prevent exposure of personnel to a sudden blast of air, water or other projectiles from the valve. Ear protection may also be required when testing high pressure valves. 250 200 Vesswl Size Ft3 150 100 50 005102530 Orifice size (in2) 1520 7% Blowdown 10% Blowdown WSIL - 199 Introduction to the Design and Maintenance.indd 10WSIL - 199 Introduction to the Design and Maintenance.indd 101/22/2009 4:54:33 PM1/22/2009 4:54:33 PM